Evaluation of Peripheral Dose for Varian Trilogy Linear Accelerator

Objective To measure and evaluate the peripheral dose(PD) for Trilogy linear accelerator in different setup condition and investigate the feasibility of the diode dosimetric system to measure the peripheral dose. Methods Peripheral dose were measured using a CC13 ionization chamber and the diode dosimetric system in a set of solid water phantom. Measurements were performed for different depths, eld sizes, physical and virtual wedge, radiation beam energy and up at distance of 1cm to 31cm beyond the eld edges. PD is separated into PD leakage and PD scatter by measure peripheral dose with or without scattering phantom. CRIS phantom was used for this research with the diode dosimetric system at the interest points of the breast, thyroid, and lens. Results All the measure data were normalized to isocenter. The measured PD decreases exponentially as a function of distance up to 31cm from the edge. PD shows no signicant relevant to depth and it increases with the increased eld size. As the physics wedge angle increase, PD increases about 1%, but enhanced dynamic wedge decreased 2-3% compared with open eld. As the beam energy increase, PD decreased. All PD data difference less than 1% between CC13 ionization chamber and diode. The PD of CRIS phantom for Volume Modulated ARC Therapy (VMAT) is minimum and the mean dose for breast (cid:0) thyroid and lens is 6.72 mGy (cid:0) 2.90 mGy and 2.37 mGy respectively. Conclusion The diode dosimetric system provides an sucient assessment in peripheral regions of 6MV X-ray beam. PD of eld size depth beam energy PD helpful


Background
In Radiation Therapy, treatment target will be irradiated, spontaneously, organs surrounded receive small amount of radiation dose inevitable. The small amount of radiation dose received are called peripheral dose(PD).Peripheral dose will increase the advent of long-term complications. For example: Primates cataract's threshold dose is 3Gy and for sexual gland is 2Gy,Dörr and Herrmann found that : The majority of secondary carcinoma were within 5 cm margin from the radiation portal edge, this correlated to regions receiving less than 6Gy radiation dose. This research is to determine the peripheral dose(PD) for Varian Trilogy accelerator and investigate the ability of the diode dosimetric system to accurately measure the PD.

Phantom scan:
Two sets of 40cm*40cm*20cm water equivalent phantom is used for PD measurement. The phantom anterior is used for 'scattering phantom' and the posterior one is used for 'measurement phantom' (as show in Figure 1a). All phantoms mentioned above CT scanned by Philips big bore CT scan system, with 5mm CT scan slice thickness. All scan images were imported to Eclipse 8.6 treatment plan system.

Experiment 1,constitution of peripheral dose:
Irradiate the scattering phantom with beam energy of 6MV, gantry angle at 0 degree and eld size is set to 20cm*20cm,isocenter irradiation with depth of 10cm, isocenter dose is set at 100cGy.Measure the peripheral dose at 1cm, 2cm, 3cm, 4cm, 7cm, 10cm, 13cm, 16cm, 19cm, 22cm, 25cm, 28cm, 31cm outside the edge of eld size by CC13 ionization chamber and Diode ionization chamber respectively. All the data acquired by this setup is de ned as 'PD all '.
Remove the 'scattering phantom' and measure the PD at the same setup as above, de ne all the acquired data as 'PD leakage ' and the differences between 'PD all ' and 'PD leakage ' as 'PD scatter '. All the peripheral dose are normalized to isocenter.

Experiment 2, relationship between irradiation depth and PD:
Under the same setup as Experiment 1, Measure all the PD data at 3cm, 10cm, 15cm depth.

Experiment 3, relationship between eld size and PD:
Under the same setup as Experiment 1, change the eld size at 10cm*10cm, 20cm*20cm, 30cm*30cm and measure all the PD data.

Experiment 4, Effects of wedge on PD:
Under the same setup as Experiment 1, add physical wedge and virtual wedge separately, which is w15, w45, vw15, vw45, measure PD as Experiment 1.

Experiment 5, relationship between beam energy and PD:
Under the same setup as Experiment 1, change the beam energy to 6MV and 18MV, Measure all the PD data as Experiment 1.

Experiment 6, PD measurement on CRIS phantom:
Hybrid treatment plan of postoperative patients with cervical cancer to CRIS tissue equivalent phantom, design VMAT, FF-IMRT step & shoot and FF-IMRT sliding window plan for the patient target, where VMAT plan consists of 2 Arc, FF-IMRT plan consists of 7 radiation elds and level of step shoot is 10 [5] . The dose measurement organs are breast, thyroid and lens with and without Lead shielding (0.5 Pb).The distance for three organs of interest to isocenter are 35cm, 51cm and 62cm and the depth are 3cm, 2cm and 1cm respectively. All doses are normalized to irradiation isocenter dose. Results 1. PD distribution at isocenter of beam energy of 6MV and 20cm*20cm eld size is shown in gure 2, 'PD all ' ,'PD leakage ' and 'PD scatter ' decrease exponentially from 13.41% to 0.25% at the distance 1cm to 31cm from eld edge. Measurement of CC13 ionization chamber and Diode ionization chamber shows good conformity and the maximum deviation is less than 1%. The relationship between 'PD leakage ' and 'PD scatter ' is shown in gure 3. 'PD scatter ' dominant in near eld edge area while 'PD leakage ' dominant remoteness area. The junction point of these two dominances is at 7cm approximately where the 'PD leakage ' and 'PD scatter ' are equal.
2. Relationship between depth and dose distribution is shown in gure 4. Depth show small impact on PD distribution, the deviation is less than 1% at different depth.
3. Relationship between eld size and dose distribution is shown in gure 5. As eld size increases, PD value increases signi cantly.  Discussion This research investigate the effect of radiation depth, eld size , wedge and beam energy on peripheral dose distribution and discuss the constitution of peripheral dose. We also discussed the feasibility of using diode system to detect peripheral dose and measure breast,thyroid and lens dose using CRIS phantom.
Gopiraj Annamalai et al [6] found that: as depth increases, the peripheral dose increase. At setup of eld size 20cm*20cm,distance from edge is 1cm at 1.5cm, 5cm and 10cm depth, the peripheral dose is: 7.8%, 10% and 16%. They show different results from us, which are 13% consistent at different depths. The reason of this difference is unsure but it may cause by the precision of position and the choice of measurement detector in spite of eld size changes in MLC for different measurement. These reasons may cause differences especially in measurement near the eld edge. R. Balasubramanian [7] found that: PD distribution distance at 5cm -20cm from eld edge in different depth is approximately equal shows similar result with us.
Our results show that as eld size increases, phantom scatter factor increase and PD increases accordingly. Virtual wedges changes dose distribution by moving jaw, when jaw moving, it blocks parts of scattering radiation generated from collimator and the volume of phantom irradiated are decrease as jaw moving. Accordingly, phantom scatter factor decrease and then the peripheral dose decreases.
Radiation beam will generate Compton effects with phantom, and with beam energy increases, backscatter electron will have more tendency to scatter in front direction. In this case, lower the peripheral dose in more distance area from eld edge. And also, in the same dose condition, higher energy radiation beam requires less MU than that of lower energy radiation beam which cause lower the collimator scatter factor in high energy radiation beam. These two reason mentioned above caused higher energy radiation beam have less peripheral dose than lower energy radiation beam. R. Balasubramanian et al. found that: in 15MV and 6MV beam energy, for peripheral dose at 5cm distance from eld edge, the peripheral dose is 3.42% and 3.07% [7] , which have similar result with us.  [14] . This research has mentioned that lead shielding can reduce breast, thyroid and lens' radiation dose. It shows a little difference than Ming X Jia et al.'s results [15] .The reason of the differences maybe the size difference of radiation target, the distance differences of organs of interest and radiation target.

Conclusion
In conclusion, radiation eld size, wedge, radiation beam energy and etc may effects peripheral dose. To lower the chance of complication of surrounding healthy tissue, we recommend using shielding technique. Diode system and CC13 ionization chamber has good conformity, and is feasible in clinic for peripheral dose measurement. In clinical setup, Organs of interest's radiation dose are related to radiation technique, target eld size, and need more analysis.

Declarations 1. Ethics approval and consent to participate
This manuscript doesn't report on or involve the use of any animal or human data or tissue so that this section is not applicable.

Consent for publication
This manuscript does not contain data from any individual person, hence this section is not applicable.

Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Figure 1
Sagittal plane of water equivalent phantom (1a) and CRIS phantom (1b). Relationship between depth and dose distribution Figure 5 Relationship between eld size and dose distribution PD distribution at different beam energy